1. Field of the Invention
The present invention relates to a motor control device which performs current vector control on a synchronous motor, and in particular, relates to a motor control device performing a current vector control on a permanent magnetic synchronous motor (PMSM) possessing saliency, on the basis of a q-axis current command and a d-axis current command.
2. Description of the Related Art
As a control method for a three-phase alternating current synchronous motor (also referred to simply as “synchronous motor” in the following) including a permanent magnet, there is known current vector control using d-q coordinate control system, as described, for example, in the document: “Design and control of an embedded magnetic synchronous motor” by Youji Takeda, Nobuyuki Matsui, Shigeo Morimoto, and Yukio Honda, Corporation Ohm, first edition, first issue, pages 17 to 27 and pages 38 to 46, 2001. In the d-q coordinate control system, a direction (i.e., a center axis of a permanent magnet) of a magnetic pole of a rotor is set as a d axis, and an axis (an axis between the permanent magnets) electrically and magnetically perpendicular to the d axis is set as a q axis. In this case, from a torque command, a q-axis current command and a d-axis current command are generated, and the q-axis current command and the d-axis current command are used to make a current flow through an appropriate excitation-phase winding in accordance with a magnetic pole position of the rotor of the synchronous motor. Thereby, desired torque depending on a torque command is generated in the synchronous motor.
For example, as described in Japanese Laid-open Patent Publication No. 2013-226033, there is a motor control device that limits a q-axis current command in accordance with a torque command and a speed command, and calculates a d-axis command from the speed command and the q-axis current command so that a processing speed is made high without using a complicated calculation equation.
Further, for example, as described in Japanese Laid-open Patent Publication No. 2012-249459, there is a motor control device in which inductance information, in which inductances of a d axis and a q axis changing in accordance with a value of a current flowing through a synchronous motor are related to a current value, is set in advance, and when current commands of the d-axis and the q-axis are generated, the d-axis current command and the q-axis current command are generated by using inductances of the d-axis and the q-axis corresponding to a value of a current that can be considered to flow through the synchronous motor by already output current commands of the d-axis and the q-axis.
Furthermore, for example, as described in Japanese Laid-open Patent Publication No. 2013-85407, there is a control method of performing control of maximum torque/current to adjust a current phase in order to prevent torque sag on a high-load side in a motor in which a difference exists between inductances of a d-axis direction and a q-axis direction, and accomplish torque improvement and miniaturization of the motor.
In a case of a permanent magnetic synchronous motor that does not have saliency, attractive force and repulsive force between a pole of a rotating magnetic field and a magnetic pole of a permanent magnet of a rotor generates magnet torque. When the number of pole pairs of the synchronous motor that does not have saliency is Pn, an interlinkage magnetic flux of the permanent magnet is Ψa, and a q-axis current that flows through an excitation-phase winding of the synchronous motor is Iq, magnet torque TM is expressed as in the equation 1.TM=Pn×Ψa×Iq  (1)
For an input torque command TCMD, a q-axis current command Iq* is generated on the basis of the equation 2 obtained by modifying the equation 1, a d-axis current command Id* is further generated from the q-axis current command Iq*, and these q-axis current command Iq* and d-axis current command Id* are used so that the synchronous motor that does not have saliency can be controlled.
                              I          q          *                =                              T            CMD                                              P              n                        ×                          Ψ              a                                                          (        2        )            
In a case of a permanent magnetic synchronous motor having saliency, reluctance torque as well as magnet torque due to a permanent magnet is generated. The reluctance torque is torque generated by only attractive force between a pole by a rotating magnetic field of a stator and a salient pole of a rotor. In the synchronous motor having saliency, since a difference exists between a q-axis inductance and a d-axis inductance, torque increases by torque corresponding to reluctance torque when operation is performed by determining a d-axis current in accordance with the maximum current and an input voltage while a q-axis current is kept constant. When the number of pole pairs is Pn, a q-axis inductance is Lq[H], a d-axis inductance is Ld[H], a q-axis current flowing through an excitation-phase winding of the synchronous motor is Iq[A], and a d-axis current is Id[A], reluctance torque TR[Nm] can be expressed as in the equation 3.TR=Pn×(Lq−Ld)×Iq×Id  (3)
Accordingly, torque T[Nm] generated in the synchronous motor having saliency can be expressed as in the equation 4 by using the equation 1 and the equation 3.
                                                        T              =                            ⁢                                                T                  M                                +                                  T                  R                                                                                                        =                            ⁢                                                                    P                    n                                    ×                                      Ψ                    a                                    ×                                      I                    q                                                  +                                                      P                    n                                    ×                                      (                                                                  L                        q                                            -                                              L                        d                                                              )                                    ×                                      I                    q                                    ×                                      I                    d                                                                                                          (        4        )            
FIG. 4 is a diagram illustrating relation between a speed and torque of the rotor when current vector control of the synchronous motor having saliency is performed by generating a q-axis current command and a d-axis current command by taking into account only magnet torque. When current vector control of the permanent magnetic synchronous motor having saliency is performed by generating a q-axis current command (and a d-axis current command) in accordance with the equation 2 that takes into account only magnet torque, torque T actually generated in the synchronous motor increases by reluctance torque TR from an input torque command TCMD. Particularly, in a high speed region, torque increase by reluctance torque TR is outstanding. Even in this case, when a speed control loop is provided in the fore stage of a torque control loop, and a position control loop is provided in the fore stage of the speed control loop, a torque command is adjusted at the speed control loop in accordance with necessary torque. Accordingly, a position and a speed of the rotor of the synchronous motor can be accurately controlled. However, as described above, torque of the synchronous motor is unable to be controlled in line with a torque command, and an error is generated.
In order to avoid such a torque error, both magnet torque and reluctance torque are used to perform current vector control for the synchronous motor having saliency. In this case, for input torque command TCMD, on the basis of the equation 5 obtained by modifying the equation 4, a q-axis current command Iq* is generated.
                              I          q          *                =                              T            CMD                                              P              n                        ×                          {                                                Ψ                  a                                +                                                      (                                                                  L                        q                                            -                                              L                        d                                                              )                                    ×                                      I                    d                                                              }                                                          (        5        )            
FIG. 5 is a diagram illustrating relation between a speed and torque of the rotor when current vector control of the synchronous motor having saliency is performed by generating a q-axis current command and a d-axis current command by taking into account magnet torque and reluctance torque. Generating a q-axis current command Iq* in accordance with the equation 5 enables an input torque command TCMD to agree with torque T actually generated in the synchronous motor.
However, the equation 5 used for calculating a q-axis current command Iq* includes an actually-flowing d-axis current Id, and a d-axis current command Id* is acquired from a q-axis current command Iq* generated in accordance with the equation 5. For this reason, when a q-axis current command Iq* and a d-axis current command Id* are attempted to be simply calculated, calculation of them is circulated. In order to avoid this circular reference, a value of d-axis current Id before latest one cycle is needed for calculating a q-axis current command Iq*, and due to this, an error is still generated. Further, the equation 5 includes a division to which a load is applied for arithmetic processing, and therefore the process takes time.